Add tini to ischool & publichelath hubs

We inherit from the geospatial rocker images, which ship
with the s6 process manager. However, JupyterHub doesn't
launch them using that. We consistently use tini across
all our images instead.

Ref berkeley-dsep-infra#2878
Ref berkeley-dsep-infra#2891

deployments/ischool/image/Dockerfile

@@ -36,6 +36,7 @@ RUN apt-get update > /dev/null && \
             libssl1.1 \
             fonts-symbola \
             gdebi-core \
+            tini \
             nodejs npm > /dev/null && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/*
@@ -68,3 +69,6 @@ RUN install2.r --error --skipinstalled \
     Hmisc plm patchwork # For https://github.com/berkeley-dsep-infra/datahub/issues/2519 \
     fpp3 # For https://github.com/berkeley-dsep-infra/datahub/issues/2510 && \
     rm -rf /tmp/downloaded_packages
+
+
+ENTRYPOINT ["tini", "--"]

deployments/publichealth/image/Dockerfile

@@ -36,6 +36,7 @@ RUN apt-get update > /dev/null && \
             libssl1.1 \
             fonts-symbola \
             gdebi-core \
+            tini \
             nodejs npm > /dev/null && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/*
@@ -72,3 +73,5 @@ RUN r /tmp/r-packages/ph-142.r
 
 COPY r-packages/ph-252.r /tmp/r-packages/
 RUN r /tmp/r-packages/ph-252.r
+
+ENTRYPOINT ["tini", "--"]